A carbon nanotube (CNT) consists of a plurality of carbon atoms arranged in a tube. The carbon atoms are arrayed in a hexagonal lattice, similar to a honeycomb. A diameter of the tube is as small as a nanometer, and has unique electrical, chemical properties.
The carbon nanotube has advantageous mechanical properties, electrical selectivity, and remarkable field emission properties. The carbon nanotube may have semi-conductor properties according to arrangement of the carbon atoms, and the energy gap of the nanotube varies depending on the size of diameter of the nanotube. For these reasons, there has been increasing interest in the carbon nanotubes in electronic, biotechnology and medical fields. Additionally, electrical properties of the carbon nanotube are incomparably more beneficial than any other materials known so far. When a thin conductive layer is formed on a plastic or glass substrate by coating the substrate with the carbon nanotubes, the substrate may have high conductivity and transparency in a visible light range, and thus can be used as a transparent electrode. Such a carbon nanotube (CNT) transparent electrode is attracting attention as a substitute for an indium tin oxide (ITO) transparent electrode which is conventionally employed for a field emission display (FED) flat panel display (FPD) or a touch panel.
Especially, a CNT transparent electrode coated on a plastic substrate is more resilient to an external impact or stress than an ITO transparent electrode which is a metal oxide thin layer, and thus the electronic properties of the CNT transparent electrode are not easily changed even when the substrate is bent or folded. Accordingly, the CNT transparent electrode has been studied as a transparent electrode for a flexible display which is regarded as a next-generation display.
To utilize the carbon nanotubes as a transparent electrode, a dispersed carbon nanotube solution needs to be coated on a given base. Well known coating methods such as spray coating, deep coating, spin coating, and roll coating have been used conventionally. Coating methods described in U.S. Pat. No. 7,118,693 and U.S. Pat. No. 7,060,241 relate to coating methods generally used for coating solutions to be coated on a base.
To optimize condtutivity and transparency for a coated CNT transparent electrode, types of carbon nanotubes to be coated are selected, additives are added to enhance the condtutivity, and the thickness of the coating solution to be disposed on the base is adjusted.
However, even when the method described above is employed to the fabrication of the CNT transparent conductive layer, the CNT transparent conductive layer does not retain satisfactory transparency and condtutivity, and thus there are still a lot of aspects of the CNT transparent conductive layer to be improved.